For example, as valve train devices for motorcycle engines, there exists conventionally a valve train device having a construction in which a crankshaft sprocket provided on a crankshaft and an intermediate sprocket disposed in the vicinity of a camshaft are connected by way of a timing chain, so that an intermediate gear fixed to the intermediate sprocket meshes with a camshaft gear fixed to the camshaft (for example, refer to JP-A-6-66111).
In the case of the construction in which the intermediate gear fixed to the intermediate sprocket is brought into mesh engagement with the camshaft gear, while a construction in which timing or alignment marks on the intermediate gear and the camshaft gear are caused to align with each other is adopted as a construction for carrying out valve timing, since the construction of the intermediate gear requires that the intermediate gear has a smaller diameter than that of the intermediate sprocket, in the event that the intermediate gear is disposed behind the intermediate sprocket, the alignment mark on the intermediate gear becomes difficult to be observed visually from the outside, and this causes a problem that the valve timing work becomes difficult to be carried out when an engine is assembled.
Note that in case a construction is adopted in which an intermediate gear is disposed in front of an intermediate sprocket (for example, refer to JP-A-9-250314), while the valve timing work becomes easy to be carried out, a dimension from the camshaft gear to the cam nose, and an area surrounding the camshaft is enlarged accordingly and the torsional amount of the camshaft becomes large, leading to a problem that the valve timing control accuracy is reduced.
The invention was made in view of the problems inherent in the conventional valve train device construction, and a problem that the invention is to solve is how to provide an engine valve train device which makes the valve timing work easy to be carried out while the intermediate gear is disposed behind the intermediate sprocket and which can improve the valve timing control accuracy while avoiding the risk that the area surrounding the camshaft is enlarged.